In the fracturing of subterranean formations, a fracturing fluid having grannular insoluble particulate material suspended or slurried therein is injected down a well penetrating the formation and forced back into the formation under pressure. By this method, the formation is caused to crack or fracture thereby offering improved communication between the recoverable fluid such as oil, gas or water in the formation and the well. Fracturing is customarily conducted by employing a thickened or gelled aqueous solution which has suspended therein particles which are substantially insoluble in the liquid and the fluids of the formation at least a portion of the particles thereby being forced to lodge in the fracture created thus propping open the fractures when the fracturing pressures are subsequently released and the well put back into production.
In order to effect clean-up of the thickened, gelled fluid, such fracturing fluids typically employ a breaker system which helps break down the viscosity of the fracturing fluid thereby permitting more easy return of the fluid to the wellbore for clearing the fluid passages for the production of hydrocarbons. One known breaker system employs ammonium or alkaline metal persulfate to break the gel. However, in relatively low temperature oil and gas reservoirs (defined as from about 50.degree. to about 125.degree. F.), a breaker aid in accordance with U.S. Pat. No. 4,250,044, comprising triethanolamine is employed. The specification of U.S. Pat. No. 4,250,044, over which this invention constitutes an improvement is incorporated herein by reference.
One fracturing fluid system which is particularly desirable for use in low temperature wells because of its relatively high proppant-carrying ability and relatively excellent clean-up characteristics are solutions of polysaacharide compounds which are crosslinked with borates. Of particular interest are borate crosslinked systems employing galactomanan gums such as guar or modified guars such as hydroxypropyl guar.
As is well-known in the art, guar-based thickening systems require a length of time in which they are maintained at an acidic pH in order to effect substantially complete hydration of the guar-based materials. In order to effect borate crosslinking of the guar materials, however, the fluid pH must be basic. In typical batch-mix formulations, the guar-based fluid thickener is added to an acidic aqueous solution and includes a borate-releasing compound. Following hydration of the guar material, a pH adjusting solution such as an aqueous solution of base is added to the acidic, hydrated guar solution to shift the pH to the basic side, thereby activating the borate crosslinking mechanism.
More recently, so-called "on-the-fly" or continuous mix operations have been employed in which the pH adjusting component is dispersed in an oil carrier and added to the hydrated acidic guar solution as it is pumped into the wellbore. This method of addition of the base in a hydrophobic slurry causes delay in the release of the pH adjusting material to a point where substantial crosslinking is delayed for a period of about one-half to five minutes so that the development of substantial viscosity due to crosslinking is also delayed thereby reducing the amount of pumping horsepower required to transport the fracturing fluid through the wellbore to the formation.
The triethanolamine breaker aid of the aforementioned U.S. Pat. No. 4,250,044 is, however, incompatible with such a delayed base-release system for delayed crosslinking. Triethanolamine is a relatively strong base and, immediately upon its addition to an acidic, guar-based fracturing fluid containing a borate-releasing compound, the pH of the fluid is shifted to basic and crosslinking is initiated. The advantages in lower pumping horsepower in a delayed crosslink system are thereby lost.